A problem concerned with pneumatic power wrenches is to accomplish an increased output torque when occasionally needed, for instance to loosen screw joints which have get stuck by cross threading, corrosion, paint etc. and which therefore require an augmented torque to be loosened. Particularly when a screw joint has become stuck during tightening due misalignment or some damage on the screw joint parts the output torque level set on the power wrench will not always be enough to loosen the joint. In such cases an augmented torque output or bias action will be necessary to manage to loosen the screw joint.
In some previously described power wrenches an augmented output torque in the reverse direction has been accomplished by using a pneumatic motor designed so as to deliver a higher torque in the reverse direction than in the normal forward direction. This means that the torque available in the reverse direction for loosening difficult screw joints is higher than the normal tightening torque. However, a power wrench having such an asymmetric motor is disadvantageous in that it will provide a biased reverse action on one type of screw joints only, either right hand threaded or left hand threaded screw joints. Apart from that, such an asymmetric motor is less powerful in the tightening direction than a symmetric motor of the same size. Once designed for one type of screw joints, for instance right hand threaded screw joints, such an asymmetrically operating power wrench can not easily be altered to provide the same torque output relationship on left hand threaded screw joints. This means that there has to be available two different types of power wrenches for obtaining the same torque output characteristics, one for right hand threaded screw joints and another for left hand threaded screw joints. This is a cost related drawback not only for the power wrench supplier but also for the operator who has to have in possession two different types of power wrenches.